Traffic is expected to increase further with as mobile broadband becomes widespread. For this reason, it is necessary to ensure new radio-wave resources. However, since bands conveniently used in mobile systems, for example, bands equal to or less than 6 GHz, have already been allocated densely, a shortage state of radio waves has occurred. In order to realize new radio-wave uses, it is necessary to flexibly use the radio waves in future by using frequency bands (white spaces) available “spatially,” “temporally,” and “technically,” even when the frequencies are the allocated frequencies, while sufficiently avoiding interference in the existing works.
With such a tendency, research, development, and standardization of wireless communication systems using the white spaces have progressed in all the countries of the world. In the United State of America, an instruction to acknowledge use of the white spaces was adopted in November 2008 and work including establishment of specific technical standards and construction of databases has been carried out in Federal Communications Commission (FCC). In Europe, Office of Communications (Ofcom) has carried out research and development of new wireless technologies for the objective of utilizing the white spaces in various countries by suggesting guidelines for liberation of the white spaces. In the standardization organizations of IEEE SCC 41, IEEE802.22, IEEE802.19, and the like, efforts to standardize technologies for utilizing the white spaces to a high degree have been actively made.
In wireless communication systems utilizing the white spaces, wireless systems (secondary systems) originally not permitted to use frequencies share frequencies according to the surrounding environment in frequency bands used by existing wireless systems (primary systems) using allocated frequencies. In this scheme, only when the primary systems do not use frequency bands spatially and temporally, the secondary systems are permitted to use the frequency bands. To adjust the frequencies, databases (DBs) that store secondary system permission information used to permit the secondary systems to use the frequencies are prepared in the wireless communication systems utilizing the white spaces.
In particular, in wireless communication systems utilizing the white spaces of TVs, the fact that the TVs do not use frequencies can be confirmed in advance by databases. Therefore, transmission power of the maximum level is used in many cases.
PTL 1 suggests a method of adaptively changing the values of permissible interference power and maximum transmission power in regard to primary systems according to communication quality of the primary systems. PTL 2 suggests a method of expanding a signal transmission opportunity in a secondary wireless communication system which is a non-priority system when a base station of a first wireless communication system which is a non-priority system increases a permissible interference level of a first wireless communication system signal transmitted using a shared frequency band based on expected interference power reported from a transmission station of the second wireless communication system which is a priority system.
In the standard IEEE802.22 (NPL 1) standardized for a wireless communication system utilizing the white space of a TV, an Etiquette Mode function is introduced to determine a frequency channel which does not overlap with a frequency between adjacent cells from frequency information or a policy obtained by self-station location information or a DB. When use of a primary system is detected as in terrestrial digital broadcasting by a Spectrum Sensing function, a structure (IDRP) is provided to transfer a channel to another backup channel determined in advance. In the IEEE802.22, control is performed such that a frequency channel overlaps between adjacent cells as little as possible. However, when a frequency channel is insufficient and a frequency has to be shared between the BS and the BS installed at a distance at which mutual interference occurs, a Self-coexistence function is used. The Self-coexistence function is a function necessary in order to share a frequency channel without mutual interference even between adjacent cells in an IEEE802.22 system.
FIG. 1 illustrates a flowchart of the Self-coexistence function.
Inter-cell information such as a frequency channel already used in Power-on of a BS (F100) and Network Discovery is acquired (F101). Acquisition of a frequency channel to be used is attempted according to Etiquette Mode (F102 and F103). When there is the frequency channel to be used, the frequency channel is used to perform communication (F104 and F110). When there is no available frequency channel, the BS enters Self-Coexistence Mode and a frequency channel used by an adjacent cell is shared to be used (F108, F109, and F110). The Self-coexistence function has On-Demand Frame Contention (F106) in which only a node acquiring a transmission right can perform communication and DS/US Split Adjustment (F107) in which a ratio of downlink and uplink communication times between the adjacent cells is adjusted, and On-Demand Frame Contention and DS/US Split Adjustment are executed by self-determination of the IEEE802.22 BS.